In a phototypesetting machine, it is often desirable to selectively project images of characters along the optical projection axis which are recorded upon a plurality of discrete tracks formed upon a font disc. The selection of a particular track may be effected by providing a selection mirror mounted upon a moveable carriage, which is shifted between first and second discrete accurately defined positions corresponding to the spacial positioning of the character bearing tracks. It is highly desirable to provide a simple, inexpensive, and reliable mechanism for accurately maintaining such moveable carriage firmly against a plateway or track while mechanically biasing first and second detent members together to cause the carriage to assume either the first or the second discrete position.